But cyber weapons reportedly give Britain the best chance of deterring Russia because the West no longer has small battlefield nuclear weapons.

The Sunday Times reported that the test to “turn out the lights” in Moscow – which will give Britain more time to act in the event of war – happened during the UK’s biggest military exercise for a decade.

5,500 British troops took part in the desert exercise in Oman, where troops also practiced other war games to combat Russia’s ground forces.

A combination photograph shows the lights of the St. Basil’s Cathedral before (L) and after they were switched off for Earth Hour in Red Square in central Moscow, Russia, March 19, 2016.